I think it may be a couple of pounds cheaper this year than the £25 listed in the web link below - best check with the phone number on the website. Don't forget the rip off charges do not include parking your car which they expect you to unhitch and keep in the nearby council car park (£8 day in Summer !). Off season though if it is quiet there is usually free parking up the hill a few minutes walk away.
If you are going for a few days then it may pay you to launch after the attendant has gone home and rent a mooring bouy off Divers Down for £10 a day and take an inflatable canoe or dinghy.